Stratigraphic record of lagoonal management since Antiquity: insights from sediment core analysis and sub-bottom profiling, lagoon of Orbetello, Italy

At the end of the Bronze Age, technical improvements have made possible the rise of the first generation of major sea powers around the Mediterranean realm, such as Etruria in Italy. The Etruscan coast was fringed by large lagoons, of which only one survives today: the lagoon of Orbetello. This peculiar lagoon is held between two sand spits that connect the former island of Argentario to the mainland. A third sand spit lies in the middle of the lagoon, and supports the Etruscan city of Orbetello. Archeological investigations suggest that the lagoon was connected to the sea through a natural outlet until the 9^th century BCE, when an integrated management plan of the lagoon was implemented to optimize fishing. Subsequent closure of the natural outlet implies that lagoon management heavily subsequently relied on man-made openings to maintain the lagoon connected to the sea and nearby rivers. Today, three canals connect the lagoon to the sea. Management plans were implemented in the late 20^th Century to fight a massive phase of eutrophication driven by excess release of fertilizers. Eutrophication as responsible for extensive fish kills, and to the release of mercury in the water column. Little is known about lagoon management and the evolution of the city before the 17^th century CE, but it is clear that the wealth of the city of Orbetello and the health of its lagoon have been strongly correlated over the past three millennia.

               To track this coevolution, we conducted a coring campaign coupled to sub-bottom imaging in the very shallow (<1.5m) waters of this extensive (30 km²) lagoon. iXblue Echoes 10 000 sub-bottom profiler reveals individual layers that can be traced across the lagoon, allowing stratigraphic correlations between cores, and highlighting the environmental significance of the sedimentary facies. Architecture of the deposits reveals a pronounced transgression of the lagoon over its shallowly inward-dipping margins since Antiquity. This led to the flooding of an array of regularly-spaced Roman farms along its southern shorelines, and to a more important flooding of Etruscan structures. Lagoon level rise was driven by a combination of regional sea level rise and fluctuations in the hydraulic balance of the lagoon, controlled by the opening and closure of canals. Sub-bottom imaging reveals buried structures tentatively interpreted as docks and canals used for navigation and salinity control.

               Sediment analyses under XRF scanning, Rock Eval, and hyperspectral imaging reveals that sedimentation is marked by an alternation of black, shelly organic silty clays and decimeter-thick layers of broken shells. Radiocarbon dating indicates that the cores capture up to five millennia of sedimentation. Ongoing analysis of mercury and phosphorus content, ostracods and pollen assemblages, and chromatic pigments in the organic fractions are expected to document the links between sediment facies, eutrophication and salinity crises, and the rise and demise of land and lagoon management through the past three millennia.

               A sub-bottom profiling campaign in June 2022 is aimed at mapping the observed structure. Further coring is planned near putative man-made structures next Fall.